This invention relates to the incorporation of sodium tetraborate pentahydrate (SBP) and naturally occurring calcium sodium borate (Ulexite), which begin to release waters of hydration at 120° C. and 59° C. respectively, into lignocellulosic, lignocellulosic thermoplastic, and thermoplastic composites that require processing temperatures from about 150° C. to 300° C. Specifically it describes a processes that modifies SBP and Ulexite prior to their incorporation into the final composite product by heating to eliminate a portion of their waters of hydration followed by mixing the resultant product with glycerol mono stearate (GMS). In alternative embodiments of the invention the heating step or the mixing step of the process is deleted.
There is a very high demand for wood products. Although wood is a renewable resource, it takes many years for trees to mature. Consequently, the supply of wood suitable for use in construction is decreasing and there is a need to develop alternative materials. One alternative has been the use of lignocellulosic composites in applications which require resistance to wood-destroying organisms such as fungi and insects. This requires treatment of these composites with a wood preserving material.
Traditionally solid wood products are dipped or pressure treated with solutions of fungicides to provide resistance to fungus and mould damage. However with a composite material, the fungicide can be incorporated during its production. This approach yields a product in which the lignocellulosic composite has a constant loading of preservative throughout its thickness, strengthening its resistance to leaching and increasing the effectiveness of the preservative.
Lignocellulosic composites are formed with thermosetting resins which undergo a chemical reaction when heated causing the resin to harden. Lignocellulosic based composites include particleboard, oriented strand board (OSB), fiberboard (medium and high density), laminated strand lumber and similar products. The methods for manufacturing thermosetting lignocellulosic composites are generally well known but the specific procedure will depend on the cellulosic material and the type of composite desired. However, generally the lignocellulosic material is processed into fractions or particles of appropriate size, which may be called a furnish, mixed with an adhesive thermosetting resin and the resultant mixture is formed into the desired configuration such as a mat, and then formed, usually under heat and pressure into the final product. The process is usually a dry one; that is generally no water is added to form a slurry of the materials; however a water slurry may be used in some processes. The resin can be of a phenol formaldehyde (PF) or iso-cyanate type and can be from about 2 to as much as 25 percent by weight of the total composite.
Many attempts have been made to use three types of hydrated sodium borates (SBP), sodium tetraborate decahydrate (SBD), and disodium octaborate tetrahydrate (DOT)) in composite products as these chemicals are low in cost and mammalian toxicity and have a minimum environmental impact. However, when phenol formaldehyde resins are used as a binder, these three borates either reduce the adhesive bonding to unacceptable levels at very low boron retentions or they require special processing techniques. Knudson et al in U.S. Pat. No. 4,879,083 issued Nov. 17, 1989 recognized that the strength of waferboard was reduced to an unacceptable level when disodium octaborate tetrahydrate was used as an additive. Hsu et al in U.S. Pat. No. 5,246,652 teaches that the use of DOT or SBP requires the use of a resin that does not react readily with these borates. Hsu identifies a requirement for as a “two-stage” (novolac) phenol formaldehyde (PF) resin as opposed to the typical “one-stage” (resole) PF used in OSB production. He also states that if a resole PF resin is used the final composite must be formed using pressurized steam preferably in a self-sealing or sealed steam press.
Knudson teaches that when anhydrous borax (sodium tetraborate with no waters of hydration) can be used as an additive and preserve the bond strength. However the production of anhydrous borax starts with the decahydrate or pentahydrate forms and requires a significant amount of energy and associated cost to totally dehydrate the chemical. Neither Hsu or Knudson discuss the use of SBP when incorporated into a lignocellulosic composite that utilizes an isocyanate-based resins.
Thermoplastic lignocellulosic composites are formed with thermoplastic resins that do not react chemically when heated. The processing methods are quite different from those associated with lignocellulosic composites. Although some of these methods are well known, others are continually being developed as this composite type continues to occupy a larger share of the construction marketplace. Lignocellulosic materials such as wood, sawdust, rice hulls, and the like are added to thermoplastic compounds to achieve a wood-like composite providing reinforcement, reduced coefficient of expansion, and cost reduction. Process methods have been developed to enable blends containing materials having low bulk density (ie. powders) and poor flow characteristics to be fed at commercially acceptable rates. Blends of this type can be extruded through dies of the appropriate configuration to produce building product type shapes previously made from wood. Processing temperatures using these methods range usually range from approximately 150 to 200° C., although some processes operate at temperatures above approximately 300° C.
When lignocellulosic thermoplastic composites were first introduced, the prevailing theory was that the plastic protected the cellulose from fungal attack. However research has revealed that lignocellulosic thermoplastics are susceptible to structural damage from fungal decay and cosmetic surface impairment from mold. See Verhey, Laks, and Richer, “Laboratory Decay Resistance of Woodfiber/Thermoplastic Composites”, Forest Products Journal, September 2001, Vol. 51 p 44-50. Further, since the primary use of lignocellulosic thermoplastics is in decking and railing products which are exposed to the elements, surface degradation due to weathering is an issue. Weathering is a complex process which includes ultraviolet (UV) light interaction and aging of the materials due to exposures such as acid rain.
Degradation due to the fungal attack is a problem that threatens the material's structural integrity. Surface lightening, discoloration, and spotting caused by mold spore production and weathering is a problem since major commercial uses of lignocellulosic thermoplastic composites, including decking and railing, rely on their aesthetic appeal to compete in the marketplace.
Zinc Borate has been used successfully to provide fungal decay in lignocellulosic thermoplastic composites at relatively low levels, typically less than 1.5 percent. However zinc borate is an expensive material when compared to the cost of the plastic binder, and its addition increases the composite's total cost. SBP and Ulexite have temperature limitations that prevent their use in many of the thermoplastic processes as they start to lose their waters of hydration at approximately 120° C. and 59° C. respectively which can cause processing problems such as excessive heat buildup during mixing or extrusion. A release of moisture can cause equipment damage via overheating or require a slow down in product output to compensate for potential overheating.
Currently the lignocellousic thermoplastic composites industry is faced with two preservation needs: finding an economic method of improving resistance to fungal decay and developing a method for improving resistance to surface visual impairment caused by mold and weathering.
Thermoplastic composites which are processed at temperatures usually in excess of 160° C., and can exceed 300° C., contain no lignocellulosic material and therefore are not vulnerable to fungal decay and insect attack. However they are susceptible to surface impairment caused by mold, especially when the material is located in a dark, moist environment as well as impairment caused by weathering. The addition of the modified SBP and Ulexite described in this invention allows their incorporation at the processing temperatures encountered when manufacturing these composites. This incorporation provides a resistance to the surface impairment.
Removing a portion of the waters of hydration in SBP and Ulexite also produces other benefits. A lesser amount by weight of the chemical is required to produce a similar level of resistance to mold or fungal decay in the composite; this is beneficial as often additional material in a composite can reduce desirable properties such as bond strength. Further, eliminating additional weight from SBP and Ulexite reduces their transportation costs.